perf: sched: Fix perf crash with new is_user_task() helper

In the Linux kernel, the following vulnerability has been resolved:

perf: sched: Fix perf crash with new is_user_task() helper

In order to do a user space stacktrace the current task needs to be a user
task that has executed in user space. It use to be possible to test if a
task is a user task or not by simply checking the task_struct mm field. If
it was non NULL, it was a user task and if not it was a kernel task.

But things have changed over time, and some kernel tasks now have their
own mm field.

An idea was made to instead test PF_KTHREAD and two functions were used to
wrap this check in case it became more complex to test if a task was a
user task or not[1]. But this was rejected and the C code simply checked
the PF_KTHREAD directly.

It was later found that not all kernel threads set PF_KTHREAD. The io-uring
helpers instead set PF_USER_WORKER and this needed to be added as well.

But checking the flags is still not enough. There's a very small window
when a task exits that it frees its mm field and it is set back to NULL.
If perf were to trigger at this moment, the flags test would say its a
user space task but when perf would read the mm field it would crash with
at NULL pointer dereference.

Now there are flags that can be used to test if a task is exiting, but
they are set in areas that perf may still want to profile the user space
task (to see where it exited). The only real test is to check both the
flags and the mm field.

Instead of making this modification in every location, create a new
is_user_task() helper function that does all the tests needed to know if
it is safe to read the user space memory or not.

[1] https://lore.kernel.org/all/20250425204120.639530125@goodmis.org/